1. Field of the Invention
The present invention relates to a holographic optical element, an optical pickup apparatus using the same and an optical recording medium drive having the same.
2. Description of the Background Art
An optical pickup apparatus used as an optical recording medium drive such as an optical disk drive uses a laser beam for recording/reading information to/from an optical recording medium such as an optical disk or for detecting servo signals.
The servo signals are categorized into a focus error signal representing the focal shift of a condensed spot of a laser beam on the optical recording medium, and a tracking error signal representing the shift of the condensed spot from a track on the optical recording medium.
The focus error signal is often detected by the astigmatism method. Meanwhile, the tracking error signal is often detected by the three-beam method when the medium is a ROM optical disk such as a CD (Compact Disk) and a CD-ROM (CD-read only memory). In the case of a recordable optical disk such as a CD-R (CD-recordable) and a CD-RW (CD-rewritable) having no information recorded and therefore having no pit, the three beam method cannot be applied, and the push-pull method or the differential push-pull method which will be described is employed.
FIG. 20 is a schematic view of a conventional optical pickup apparatus for a recordable optical disk. A semiconductor laser device 302 emits a laser beam (light beam). The light beam emitted from the semiconductor laser device 302 is divided by a diffraction grating 303 into three light beams including a main light beam and two sub light beams which are made into parallel beams by a collimator lens 304. The three light beams transmitted through the collimator lens 304 are transmitted through a beam splitter 305, and condensed by an objective lens 306 as a main spot and sub spots positioned on both sides thereof on the recording medium surface of an optical disk 301.
The objective lens 306 is supported by an actuator 310 movably in the radial direction of the optical disk 301 for tracking operation and movably in the direction perpendicular to the recording medium surface of the optical disk 301 for focus operation.
The three returned light beams (reflected light beams) from the optical disk 301 are transmitted through the objective lens 306, reflected by the beam splitter 305, transmitted through an objective lens 307 and a cylindrical lens 308 and detected by a photodetector 309. At the time, based on the combination of the objective lens 307 and the cylindrical lens 308, the three returned light beams are provided with astigmatism for focus error detection.
FIG. 21 is schematic plan views of an example of the photodetector 309 in FIG. 20. FIG. 21(a) shows the state of condensed spots when the optical disk 301 is too near to the objective lens 306. FIG. 21(b) shows the state of condensed spots when the optical disk 301 is in the position of the focal point of the objective lens 306. FIG. 21(c) shows the state of condensed spots when the optical disk 301 is too far from the objective lens 306.
As shown in FIG. 21, the photodetector 309 includes a four-segment photodetection part 160 provided in the central part thereof, and two-segment photodetection parts 161 and 162 provided on both sides of the four-segment photodetector 160. The four-segment photodetector 160 is divided into four photodetection parts A, B, C and D. The two-segment photodetector 161 is divided into two photodetection parts E1 and E2, and two-segment photodetection part 162 is divided into two photodetection parts F1 and F2. The main light beam among the three returned light beams from the optical disk 301 comes into the center of the four-segment photodetection part 160, and the two sub light beams among the returned light beams from the optical disk 301 come into the center of the two-segment photodetection parts 161 and 162, respectively.
As the distance between the optical disk 301 and the objective lens 306 changes, the focal point of the returned light beams change, and the shapes of the condensed spots on the four-segment photodetection part 160 and the two-segment photodetection parts 161 and 162 in the photodetector 309 change as shown in FIG. 21.
When the optical disk 301 is too near to the objective lens 306, as shown in FIG. 21(a), the condensed spot S is in an elliptical shape having its major axis direction set in the direction connecting the center of the photodetection part B and the center of the photodetection part D.
When the optical disk 301 is in the position of the focal point of the objective lens 306, as shown in FIG. 21(b), the condensed spot S is in a circular form in the center of the photodetection parts A, B, C and D.
When the optical disk 301 is too far from the objective lens 306, as shown in FIG. 21(c), the condensed spot S is in an elliptical shape having its major axis direction set in the direction connecting the center of the photodetection part A and the center of the photodetection part C.
Therefore, using output signals PA, PB, PC and PD from the photodetection parts A, B, C and D, respectively in the four-segment photodetection part 160, a focus error signal FES is obtained from the following expression:FES=(PA+PC)−(PB+PD)  (1)
The focus error signal FES according to the above expression has a negative value when the optical disk 301 is too near, has the value of zero when the optical disk 301 is in a good focus state, and has a positive value when the optical disk 301 is too far. Thus, the direction of the shift of the optical disk 301 from the position of the focal point can be determined based on the sign of the focus error signal FES.
The focus error signal FES is fed back to the actuator 310, and the objective lens 306 is moved in the direction perpendicular to the optical disk 301 to correct the condensed state on the optical disk 301.
When the optical axis of the semiconductor laser device 302 is inclined, a deviation is caused in the light intensity distribution in the condensed spot at the photodetector 309 in the focused state. According to the astigmatism method using the four-segment photodetection part 160 described above, the deviation in the light intensity distribution in the condensed spot caused by the inclination of the optical axis of the semiconductor laser device 302 is less likely to cause an error in the focus error signal FES.
FIG. 22 is views for use in illustration of the principle of tracking servo control by the push-pull method or the differential push-pull method. In the left part of the FIG. 22(a) to (c), the positional relation between the optical disk 301 and the objective lens 306 is shown, while in the right part, the light intensity distribution in the far-field pattern near on the photodetector 309 is shown. In the left part of FIG. 22(a) to (c), the main light beam is denoted by a solid line, while the sub light beams are denoted by broken lines.
In the recordable optical disk 301 such as a CD-R, a pre-groove (groove) 600 used for detecting a tracking error is formed on the recording medium surface. The pre-groove 600 includes raised land parts 601 and recessed groove parts 602. Information is recorded to the land part 601. The tracking error signal represents the shift of the main light beam relative to the land part 601.
The far-field pattern 700 of the main light beam among the returned light beams has a double-humped intensity distribution by the diffraction effect of the light at the edge of the land part 601 or the groove part 602.
As shown in FIG. 22(b), when the condensed spot of the main light beam on the optical disk 301 is positioned in the center of the land part 601, the far-field pattern 700 of the main light beam has a symmetrical, double-humped intensity distribution. In this case, the light intensity at the two photodetection parts A and D is equal to the light intensity at the other two photodetection parts B and C in the four-segment photodetection part 160.
As shown in FIG. 22(a), when the condensed spot of the main light beam on the optical disk 301 shifts to the right relative to the land part 601, the far-field pattern 700 of the main light beam has an asymmetrical, double-humped intensity distribution. In this case, the light intensity at the two photodetection parts A and D is higher than the light intensity at the other two photodetection parts B and C in the four-segment photodetection part 160.
As shown in FIG. 22(c), when the condensed spot of the main light beam on the optical disk 301 shifts to the left relative to the land part 601, the far-field pattern 700 of the main light beam has an asymmetrical, double-humped intensity distribution. In this case, the light intensity at the two photodetection parts B and C is higher than the light intensity at the other two photodetection parts A and D in the four-segment photodetection part 160.
Therefore, if the four-segment photodetection part 160 to detect the main light beam of returned light is considered as a two-segment photodetection part including two divisional parts, i.e., the photodetection parts A and D and the photodetection parts B and C, using the output signals PA, PB, PC and PD from the photodetection parts A, B, C and D, the tracking error signal TES according to the push-pull method can be obtained from the following expression:TES=(PA+PD)−(PB+PC)  (2)
The tracking error signal TES according to the expression is zero when the condensed spot of the main light beam on the optical disk 301 is positioned in the center of the land part 601. The signal TES has a positive value when the condensed spot of the main light beam on the optical disk 301 is shifted to the right from the center of the land part 601, and has a negative value when the condensed spot of the main light beam on the optical disk 301 is shifted to the left from the center of the land part 601.
However, if the optical disk 301 is inclined, the far-field pattern 700 on the four-segment photodetection part 160 has an asymmetrical, double-humped intensity distribution even though the condensed spot of the main light beam on the optical disk 301 is positioned in the center of the land part 601.
In an optical pickup apparatus which does not drive an optical system including the objective lens 306 as a whole, when only the objective lens 306 is moved for tracking servo control, the far filed pattern 700 on the four-segment photodetection part 160 has an asymmetrical, double-humped intensity distribution even though the condensed spot of the main light beam on the optical disk 301 is positioned in the center of the land part 601.
In these cases, an apparent tracking error is caused. The apparent tracking error is called “tracking error offset.”
Therefore, in order to reduce the tracking error offset caused when the optical disk 301 is inclined or the objective lens 306 is moved, the differential push-pull method is applied. In tracking servo control according to the differential push-pull method, two-segment photodetection parts 161 and 162 on both sides of the four-segment photodetection part 160 are used.
As shown in FIG. 22, the condensed spots by the sub light beams obtained by the diffraction grating 303 shown in FIG. 20 are formed at groove parts 602 on both sides of the land part 601. Thus, the far-field patterns 701 and 702 of the sub light beams among the returned light beams have a double-humped intensity distribution on the two-segment photodetection parts 161 and 162, respectively by the diffraction effect of the light at the edge of the land part 601 or the groove part 602.
As shown in FIG. 22(a), when the optical disk 301 is shifted to the left, the light intensity at the photodetection part E2 is higher than the light intensity at the photodetection part E1 in the two-segment photodetection part 161 and the light intensity at the photodetection part F2 is higher than the light intensity at the photodetection part F1 in the two-segment photodetection part 162.
As shown in FIG. 22(c), when the optical disk 301 is shifted to the right, the light intensity at the photodetection part E1 is higher than the light intensity at the photodetection part E2 in the two-segment photodetection part 161 and the light intensity at the photodetection part F1 is higher than the light intensity at the photodetection part F2 in the two-segment photodetection part 162.
Thus, the asymmetry of the light intensity distribution in the far-field patterns 701 and 701 by the sub light beams is reversed from the asymmetry of the light intensity distribution in the far-field pattern 700 by the main light beam. As a result, using the output signals PA, PB, PC and PD from the photodetection parts A, B, C and D and output signals PE1, PE2, PF1 and PF2 from the photodetection parts E1, E2, F1 and F2, the tracking error signal TES according to the differential push-pull method can be obtained from the following expression:TES=(PA+PD)−(PB+PC)−k {(PE1+PF1)−(PE2+PF2)}  (3)
where k is a coefficient set so that the tracking error offset is initially zero. Thus, according to the differential push-pull method, the tracking error offset can be compensated.
In recent years, attempts have been carried out into reduction of the size of the optical pickup apparatuses for recordable optical disk using a holographic optical element, similarly to the case of the conventional optical pickup apparatus for reproduction.
FIG. 23 is a schematic view of an optical pickup apparatus having a transmission-type holographic optical element disclosed by Japanese Patent Laid-Open No. 3-76035.
In FIG. 23, the radial direction of an optical disk 501 is the X-direction, the track direction of the optical disk 501 is the Y-direction and the direction perpendicular to the disk surface of the optical disk 501 is the Z-direction.
The optical pickup apparatus shown in FIG. 23 includes a holographic unit 520 and an objective lens 511.
A heat sink block 504 is provided on a stem 502, a sub mount 505 is attached to a side surface of the heat sink block 504 and a semiconductor laser device 506 is attached on the sub mount 505. A photodetector 507 is provided on an upper surface of the heat sink block 504. A cap 503 is provided to surround the heat sink block 504. At an opening at the upper surface of the cap 503, a holographic optical element 508 is provided. At a lower surface of the holographic optical element 508, a diffraction grating 509 is provided, and at an upper surface of the holographic optical element 508, a holographic surface 510 is formed.
The semiconductor laser device 506 emits a laser beam (light beam) in the Z-direction. The light beam emitted from the semiconductor laser device 506 is divided into three light beams, i.e., a 0th order diffracted light beam (main light beam), a +1st order diffracted light beam (sub light beam) and a −1st order diffracted light beam (sub light beam) by the diffraction grating 509 within a plane substantially including the Y- and Z-directions, and the light beams are transmitted through the holographic surface 510.
The three light beams transmitted through the holographic surface 510 are condensed by the objective lens 511 as a main spot and sub spots positioned on both sides of the main spot on the optical disk 501. The objective lens 511 is supported by an actuator 512 movably in the X-direction for tracking operation and in the Z-direction for focus operation.
The three returned light beams (reflected light beams) from the optical disk 501 are diffracted within a plane substantially including the X- and Z-directions by the holographic surface 510, and detected by the photodetector 507. As shown in FIG. 23, the holographic surface 510 has an asymmetrical pattern, and provides the three returned light beams from the optical disk 501 with astigmatism.
In the optical pickup apparatus in FIG. 23 using the holographic optical element 508, the operation described with reference to FIG. 21 and FIG. 22 can similarly be achieved. In this case, the photodetector 507 has a four-segment photodetection part 160 and two two-segment photodetection parts 161 and 162 similarly to the case of the photodetector 309 in FIG. 21.
Thus, using the holographic optical element 508, the optical system can be formed into a unit since the semiconductor laser device 506 and the photodetector 507 are used in a chip form. As a result, the size of the optical pickup apparatus can be reduced.
In the semiconductor laser device 506, however, the lasing wavelength varies depending upon the ambient temperature. The variations in the lasing wavelength cause the diffraction angle of returned light beams on the holographic surface 510 to change.
FIG. 25 is schematic plan views showing the movement of the condensed spots on the photodetector 507 by variations in the lasing wavelength of the semiconductor laser device 506 in the optical pickup apparatus in FIG. 23.
At the time of adjustment, as shown in FIG. 25(b), the condensed spot S of the main light beam is positioned in the central part of the four-segment photodetection part 160. When the lasing wavelength of the semiconductor laser device 506 is shorter at a lower ambient temperature, as shown in FIG. 25(a), the condensed spot S on the four-segment photodetection part 160 moves in the direction opposite to the diffraction direction (−X-direction). Conversely, when the lasing wavelength of the semiconductor laser device 506 is longer at a higher ambient temperature, as shown in FIG. 25(c), the condensed spot S on the four-segment photodetector 160 moves in the same direction as the diffraction direction (+X-direction). Consequently, the level of the focus error signal FES is lowered, and the detection accuracy of the focus state is lowered.